When manufacturing various parts it is desirable to test the parts themselves or certain specimens from these parts for mechanical strength. For example, in the airplane industry, some of the airplane parts are made out of composite materials. It is desirable to test specimens of these parts for compression strength.
There are large machines available to accomplish this testing. For example, during compression testing, the specimen, such as a flat piece of laminate (sometimes referred to as a "coupon"), is supported by a conventional test specimen holder (also referred to as a "fixture") between the grips of the testing machine. The machine compresses the coupon in a lengthwise direction until it fractures whereupon the machine provides a readout of the force required to fracture the coupon. Support for the coupon along its lengthwise axis is required during compression testing to prevent Euler column buckling. However the coupon should be supported in a manner so that sublaminate buckling of the specimen or any natural failure mode of interest is not restricted. In addition, during the compression testing it is sometimes desirable to attach a extensometer (or strain gauge) to the coupon to measure the amount of deformation of the coupon during compression.
Existing -compression fixtures support t faces of the test specimen. While these fixtures restrict the Euler column buckling they also restrict the valid sublaminate buckling failure mode. Specimen designs based on stable column sections have been used to measure compression properties of composites. These specimens have very restrictive application ranges and are not suitable for general applications.
Furthermore, these conventional fixtures have a large contact area with the test specimen which allows for some of the test load to be transferred to the fixture itself resulting in an erroneous measurement of the coupon's compression strength in a nonconservative manner. Also, an opening must be cut into the fixture to allow for open and filled hole compression tests. This makes it difficult to perform measurements of strain in the vicinity of the opening. And furthermore, conventional fixtures cannot be easily modified for compression testing where it is desirable to support only one lengthwise extending edge of the test specimen.
Other conventional support devices have been disclosed. For example, U.S. Pat. No. 683,184 by Rockwell discloses a clamp having four rectangularly arranged blocks connected together in pairs by compression and expansion screws. In addition, U.S. Pat. No. 2,350,060 by Montgomery and U.S. Pat. No. 2,368,900 by Templin disclose compression testing devices for thin specimens wherein the devices each include a pair of T-shaped jaws having small diameter rollers to engage the side surfaces of the specimen. And, U.S. Pat. No. 4,840,070 by Ralfs et al discloses a laminate compression tester which includes a pair of adjustable stabilizing jaws having end segments which engage specimen grips of a testing machine.